Why are Exhaust ports 10% of Intakes?
This is easier to read, hopefully
Darin MorganShow Guest
The 75% intake to exhaust flow ratio is NOT correct and never was. I think some engineer pulled this out of thin air.
The engines RPM range, size and intended use as well as other design criteria will be the deciding factor as to valve size and placement but trying to hit some preconceived flow ratio with total disregard for other more important tuning variables is not an intelligent or well thought out approach. I will admit that the industry is full of these little snippets of pseudo wisdom but one should not believe everything they hear. When you try and optimize the valve area in an unlimited engine combination you always sacrifice exhaust valve size for intake valve size. Its much easier to cam and manipulate the exhaust system to evacuate the cylinder ( dynamic blow down and cam shaft events) than it is to manipulate the intake tract. Current Pro Stock and Comp Eliminator engines have intake to exhaust (size in diameter) ratios of 70% and flow ratios of only 58% to a maximum of 61%. These engines are producing 2.78HP/CID at over 5800fpm piston speed so the that kind of shatters the long held belief that the exhaust should flow 75%-80% of the intake. Could we make more power if the exhaust flowed more. No, we tried that. Could we make more power if we increased the exhaust size and kept the same discharge coefficient? Yes it does but there isn't any room left for a larger valve because we just stuck this big intake in there so its really a mute piont. I know thats a simplistic explanation of what is going on but I think it drives the piont home.
Darin Morgan
-Induction Research and Development
-EFI Calibration and Tuning
Reher Morrison Racing Engines
1120 Enterprise Place
Arlington Texas 76001
Phone 817-467-7171
Cell 682-559-0321
Reher Morrison Racing Engines
This is easier to read, hopefully
Darin MorganShow Guest
The 75% intake to exhaust flow ratio is NOT correct and never was. I think some engineer pulled this out of thin air.
The engines RPM range, size and intended use as well as other design criteria will be the deciding factor as to valve size and placement but trying to hit some preconceived flow ratio with total disregard for other more important tuning variables is not an intelligent or well thought out approach. I will admit that the industry is full of these little snippets of pseudo wisdom but one should not believe everything they hear. When you try and optimize the valve area in an unlimited engine combination you always sacrifice exhaust valve size for intake valve size. Its much easier to cam and manipulate the exhaust system to evacuate the cylinder ( dynamic blow down and cam shaft events) than it is to manipulate the intake tract. Current Pro Stock and Comp Eliminator engines have intake to exhaust (size in diameter) ratios of 70% and flow ratios of only 58% to a maximum of 61%. These engines are producing 2.78HP/CID at over 5800fpm piston speed so the that kind of shatters the long held belief that the exhaust should flow 75%-80% of the intake. Could we make more power if the exhaust flowed more. No, we tried that. Could we make more power if we increased the exhaust size and kept the same discharge coefficient? Yes it does but there isn't any room left for a larger valve because we just stuck this big intake in there so its really a mute piont. I know thats a simplistic explanation of what is going on but I think it drives the piont home.
Darin Morgan
-Induction Research and Development
-EFI Calibration and Tuning
Reher Morrison Racing Engines
1120 Enterprise Place
Arlington Texas 76001
Phone 817-467-7171
Cell 682-559-0321
Reher Morrison Racing Engines